In a field of color display apparatus, there is an increasing tendency for small thin low-power consuming ones to be demanded, and pocket-size television sets using liquid crystal as a shutter have come as goods to public notice.
As shown in FIG. 18, a thin full-color display apparatus used in a conventional pocket-size television set includes shutter means 100 in the form of a matrix of liquid crystal cells C, a light source 101 disposed behind the shutter means, and filter means 102 disposed before the shutter means and including a repeat of a red transparent filter R, a green transparent filter G and blue transparent filter B arranged in order in correspondence to the liquid crystal cells. By controlling voltages applied to the respective liquid crystal cells in accordance with image information, quantities of light from the light source and passing through the liquid crystal cells are adjusted to thereby adjust the luminance and chromaticity of the respective pixels.
However, in such thin full-color display apparatus, there is the problem that contrast is not excellent due to the characteristic of the liquid crystal itself and that the angle of visual field is very narrow. In such apparatus, a light source as backlight is needed, so that there is the problem that the entire apparatus would be thick although the liquid crystal section itself is thin.
The thin-film EL elements each include a thin transparent luminous layer and has no granularity. Therefore, external incident light and light emitted within the luminous layer are not scattered, so that they cause no halation or oozing, the display is clear and provides high contrast. Therefore, they are highlighted as being used for a display or illumination unit.
The basic structure of a thin-film EL element includes a double dielectric structure which in turn includes on a transparent substrate a transparent electrode of tin oxide (SnO.sub.2) layer, etc., a first dielectric layer of tantalum pentaoxide layer, etc., a thin luminous layer of zinc sulfide (ZnS), etc., and containing manganese (Mn), etc., a second dielectric layer of tantalum pentaoxide, etc., and a rear electrode of an aluminum (Al) layer, etc., laminated in order.
The process of luminescence is as follows. If a voltage is applied across the transparent electrode and rear electrode, the electrons trapped at the interface level are pulled out and accelerated by an electric field induced within the luminous layer so that they have energy enough to strike orbital electrons in Mn (the luminescent center) to thereby excite same.
When the excited luminescent center returns to its ground state, it emits light.
Researches in which a multicolor display panel is fabricated using thin-film EL elements have recently become popular and various researches have been made on making full color panels.
A thin-film EL element emitting white light uses a luminous layer of zinc sulfide containing praseodymium fluoride (PrF.sub.3), as disclosed in Yoshihiro Hamakawa et al., The Institute of Electronics and Communication Engineers of Japan Technical Research Report, CPM 82-10, 1982.
As shown in FIG. 19, the thin-film EL element using the luminous layer of zinc sulfide containing praseodymium fluoride has peaks at about 500 and 650 nm in the emission spectrum. The rays of light at 500 and 650 nm are in complementary-color relationship to each other and show as if they were white light. However, the light does not contain three primary colors, so that it cannot be used for full color display.
A thin-film EL element having such structure is all transparent except for its rear electrode. Thus external incident light is reflected by the rear electrode and the reflection interferes with the light from the luminous layer so that it does not provide a satisfactory contrast ratio and thus only display devices having low display quality would be provided.
The present invention has been made in view of such situations. It is an object of the present invention to provide a thin color display apparatus which provides high contrast and a wide angle of visual field.
It is another object of the present invention to give high dielectric strength to the thin-film EL elements of a color display apparatus.
It is a further object of the present invention to improve the contrast of the thin-film EL elements.